Multiband superheterodyne receiver



Feb. 4, 1941.

L. BABIK ET AL MULTIBAND SUPERHETERODYNE RECEIVER Filed March 17, 1938 INVENTORS LUDW/G BAB/K RUDOLF BURGHOLZ BY W ATTORNEY Patented Feb. 4, 1941 UNITED STATES PATENT OFFICE MULTIBAN D SUPERHETERODYNE RECEIVER Ludwig many,

Babik and Rudolf Burgholz, Berlin, Gerassignors to Telefunken Gesellschaft fiir Application March 17, 1938, Serial No. 196,360

In Germany April 17, 1937 1 Claim.

The invention is concerned with the oscillator circuit organization of a superheterodyne apparatus and its purpose is the elimination of certain difficulties found to arise in the oscillator circuits upon changing the wave-band. Attempts have been made in the prior art to simplify the wave-band change in the oscillator circuit by providing a single switch S in the tuning circuit as shown in Fig. 1. Thus, when receiving the medium wave-band, only the coil L3 has been short-circuited or disconnected and not the feedback or tickler coil L4. Where this scheme was used, in certain parts of the mediumwave band range, appreciable differences in the dial calibration were discovered in most sets of a given manufacturing series, and thus a disturbance in the synchronous operation of the oscillator circuit and the input circuit. Attempts have been made to obviate these discrepancies, which were found to show a very irregular shape inside the wave-band, by providing notches in the rotary condenser. However, this failed to remedy the situation and evil, for the reason that the disparities were not the same in different sets.

In a superheterodyne receiver comprising waveband switch means in the tunable circuit, though having no such switching means in the feedback circuit of the apparatus, the difficulty arising in the medium-wave-band as hereinbefore described is obviated according to the invention by the fact that the regeneration circuit or path containing the tickler coils for the various wave-bands in series is so dimensioned that its natural frequency is about twice as high as, or still higher than, the maximum oscillator or heterodyne frequency to be used. This end is attainable by a correspondingly low number of turns in the tickler coils. By choosing a stronger or closer coupling, a sufficiently high oscillator potential is obtainable.

Applicants have determined that, in the absence of switch means in the feedback circuit the natural wave of the feed-back branch came too close to the medium-wave-band or even inside the same, with the consequence that such trouble as has been described above was occasioned. The difierences observed in such disparities may be traced back to the fact that upon changes of the distance between coil L3 and coil L4, which amounts to a few millimeters, inside such tolerance limits as are conditioned by large-scale manufacture, the inductive reactance of L; was reduced in varying degrees by the short-circuited coil L3. In this way shifts in the disturbing natural wave of the feedback circuit were caused. It is easily possible to trace down the abovementioned action quantitatively by measuring and plotting the oscillator grid current throughout the entire receiving range and measurement of the frequency distortion with the switch S being opened and short-circuited.

One preferable embodiment of the idea underlying the invention which is illustrated in Figures 1 and 2 which show the circuit diagram and the positioning of the feedback means respectively consists in making the coupling relationship between the long-wave tickler coil L4 and the long-wave coil L3 pertaining to the tunable circuit as close and strong as possible, that is, by causing it to bear directly upon the grid circuit as shown, for instance, in the lower part of Fig. 2. The desired oscillator potential suflicient for high mixer or I. F. amplification is then adjusted simply by proper choice of the number of turns of the tickler coil L4 (by measurement of the grid current). By virtue of the strong coupling as suggested between coils L3 and L4, what is primarily obtained is that the number of turns of coil L4 may be made small; moreover, that with shorted coil L3 the inductance of coil L4 is still further minimized. As a consequence, in case of reception of medium waves the natural frequency of the regeneration circuit is shifted sufficiently far outside the range.

In order to preclude the chances of capacitive action upon the circuit C, L1 in receiving medium waves, it is recommendable to make the coupling between coils L2 and L1 looser, as shown in Fig. 2, though also in this instance the coupling could be made very strong. The number of turns of coil L2 also is chosen so that the desired oscillator potential will be secured. There is no critical value for the coupling, that is to say, for the distance from coils L1 and L2, so that manufacturing difficulties involving a check-up by calipers of the inter-coil spacing, 'etc., will no longer arise.

The invention maybe applied in a similar way in cases where, for instance, there is besides a short-wave band.

What we claim is:

An oscillation generator comprising a vacuum tube having at least a cathode, an anode and a grid, a tunable circuit connected between cathode and grid, said circuit including a pair of seriesconnected inductances and a shunt variable condenser capable of resonating said inductances over a certain range of frequencies, means for short-circuiting one of said inductances whereby the shunt variable condenser is capable of resonating the other inductance over a difierent range of frequencies, and a feed-back circuit connected between cathode and anode and including a pair of series-connected inductances coupled respectively to the first mentioned inductances, said feed-back circuit being so proportioned that its natural frequency is at least twice as high as the maximum oscillator frequency obtainable with the tunable circuit, and the degree of coupling between the inductance which is adapted to be short-circuited and its associated inductance in the feedback circuit being closely coupled and that between the remaining associated inductances being loosely coupled.

LUDWIG BABIK.

RUDOLF BURGHOLZ. 

